Vestas takes no chances with 7MW offshore turbine design

The unveiling of Vestas' latest turbine, a 7MW colossus built around a semi-geared drivetrain, cuts against the trend towards direct-drive transmissions for offshore machines.

Launch of the V164-7.0MW came two weeks after Alstom took the wraps off its "marinised" 6MW direct-drive machine and Siemens showcased a 2.3MW direct-drive unit devised to be scaleable to mega-class for offshore use.

The V164 represents the largest research and development (R&D) investment ever made by Vestas. It is, by all measures, massive, designed "from the first nut to the last bolt" for the extreme rigours of the North Sea environment.

The turbine, as its name suggests, has a rotor with a diameter of 164 metres - greater than the length of two Airbus A380 jets nose to nose - giving it a wind-devouring swept area of more than 21,000 square metres.

The 80-metre-long, 35-tonne carbon-fibre blades are mounted on a hub and nacelle weighing just shy of 400 tonnes. Tip height is 187 metres.

The tubular steel towers will be fabricated on a "site-specific" basis, based on a minimum hub height of 105 metres, with trussed jacket foundations foreseen for deeper water locations.

Although the giant "rotor is the motor", as Vestas Technology R&D president Finn Strøm Madsen notes, it is how the captured wind energy is transformed into electricity that the company hopes will set the V164 apart: namely, a medium-speed, semi-geared drivetrain and permanent-magnet generator (PMG).

Advocates of gearless transmission systems make their case largely around a part count. The direct- drive machine is best suited to offshore, the argument runs, due to its mechanical simplicity. Fewer components to go wrong or wear out equals less production time lost to maintenance and repair - and translates into greater reliability and power output.

Madsen turns this argument on its head. For one thing, he points out, direct-drive turbines have four times the electrical components of gearbox-driven machines, so they have their own Achilles heel; for another, geared transmission systems, whatever their frailties, are field-proven to an extent that direct-drives are not.

"Vestas is not married to one type of technology," Madsen says. "We made our choices based on proven concepts from the wind-power or other relevant industries. Striving to be a leader at sea will not be about taking chances. Deep water, rough seas, far from shore - why would you gamble with new and unproven technology? The V164 is about de-risking.

"We started out with the V164 having several development tracks, one working towards a direct-drive and one towards a geared solution. Our target was a long-lasting offshore power plant component.

"It soon became clear that if we wanted to meet the customers' expectations and [ensure the] lowest possible cost of energy and highest business-case certainty, we needed a perfect combination of innovation and proven technology, and so the choice could only be to go for a medium-speed drivetrain solution."

The rotor turns at 10-12rpm, with the gearbox stepping rotations up to 400rpm for power production by a segmented PMG with full-scale converter. Vestas is "open to discussion" as to whether to outfit the new model with its own PMGs, or buy in the units. "Our turbines, our technologies, perform out there in the real world," Madsen adds.

"The question to me is not: Why choose a geared turbine? It is: Why not choose a geared solution, when we know that it works, with very low lost production times?"

The increased energy capture by the turbine's vast rotor span and its blades' proprietary airfoil combines with a lightweight overall design (ready for service, the V164 weighs in at about 800 tonnes) to create a machine that yields 30% more output per tonne than any model now on the market.

The V164, which is designed for wind speeds from 4-25 metres per second, will churn out enough power over a year to meet the electricity demands of 6,500 homes. Keen to beef up its operational reliability, Vestas has built in risk-mitigating features.

These include "fault-tolerant" modes, so that the turbine can run at reduced output if necessary; and a "redundant component" philosophy, inspired by the aerospace industry, to avoid maintenance visits between scheduled servicing, while maintaining baseline output. "In the V164, if certain systems fail, others will take over the function and the turbine will be able to continue operating," explains Madsen.

"These turbines do their own 'health checks' using advanced diagnostics that give early warnings on potential failures, which gives us the possibility of controlled operations of the turbine until the next servicing."

This intelligence is married to "neighbouring info sharing": if a sensor fails on one turbine, it will use the data fed back by a sensor from another in the wind farm to optimise its performance.

The day after the wraps came off the V164 last week, Vestas inked a letter of intent to put the turbine through its paces at a Dong Energy test site before erecting the machines on a future commercial offshore wind farm.

Prototypes will be built this year, with trials running until 2013. Ramp-up of manufacturing, at an as-yet-unbuilt facility, will follow in 2014. The first units are expected off the assembly line in early 2015, "provided a firm order backlog is in place".

"By 2015, we are convinced the V164 will be helping to significantly narrow the gap between the cost of energy onshore and that offshore," says Madsen.